1. Field of the Invention
The present invention relates to microscopes, and more particularly to an atomic force microscope that uses a probe tip to detect the surface of a test sample by the displacement of a laser beam.
2. Description of Prior Art
The relationship of distance and relative force between the probe tip of an atomic force microscope or a scanning atomic force microscope (AFM) and the surface of a test sample to be scanned by the probe is depicted in FIG. 1. As shown, the abscissa represents the distance between the probe tip and the surface of the test sample, while the positive ordinate represents the repulsive force between the probe tip and the surface of the test sample and the negative ordinate represents the attractive force between the probe tip and the surface of the test sample. Consequently, there are two types of atomic force microscopes: one applied to an operation zone assigned with reference numeral Axe2x80x2 shown in FIG. 1 which is subject to the contact mode under repulsive force and the other one applied to an operation zone assigned with reference numeral Bxe2x80x2 shown in FIG. 1 which is subject to the tapping mode under repulsive force and attraction force.
The contact mode AFM uses a repulsive force between a probe tip and the surface of the scanned test sample. If the distance between the probe tip and the surface of the scanned test sample obtaining from scanning a probe is larger than 0 and less than d1, the scanning probe will be separated from the atom on the surface of the scanned test sample due to the repulsive force. For a tapping mode atomic force microscope, a repulsive force occurs when the distance between the probe tip and the surface of the scanned test sample is between 0 and d1. When the distance between the probe tip and the surface of the scanned test sample is larger than d1 and less than d2, the probe tip and the surface of the scanned test sample will attract each other to keep them in oscillation in a distance between 0 and d2 (like tapping).
The structure of a contact mode atomic force microscope is illustrated in FIG. 2. The probe tip 1a of the scanning probe 1 scans the surface of the test sample (not shown in the figure). The discontinuous signal generator 3 produces a discontinuous signal that will be sent to the modulating laser diode 4 to output a discontinuous laser beam. The laser beam reflects from the back side of the probe 1 to a photo detector 5 and it will then output a current signal corresponding to the intensity of the laser beam by a photoelectron conversion effect. The output current signal will be then converted into a voltage signal by a current/voltage converter 6 and calculated by a differential amplifier 7 to obtain a voltage value corresponding to the deformation of the probe 1. After entering this voltage value to a Z-axis servo controller on the piezoelectricity platform (only the seat is shown), a corresponding control command will be obtained on the basis of the control calculation principle (for example, proportional integral differential, PID) to make the piezoelectricity platform seat 2 move up and down and keep the deformation value of the probe 1 be constant. If the position of Z-axis is recorded at a specific time point and all the data are collected, the surface profile of the test sample can be obtained. Because the laser beam is a kind of pulse signal, the voltage value of the deformation of probe 1 or the position of Z-axis may be discontinuous. To obtain a smoother servo controlled and scanning image, an interpolation operation 8 is employed to compensate the signals or data where no laser beam is used. The resulting data are stored in a scanning image data storage device 9 and incorporated into a collected data point to display on a display 19 after processing by an image-processing device 18. Because the structure of a contact mode atomic force microscope uses a discontinuous laser beam, it can effectively reduce the heat deformation problem resulting from the laser beam. But it must use interpolation operation and cannot filter the interference caused, for example, by the coaxial light radiated from the charge coupling device (CCD) at the scanning probe or from a miscellaneous light source. All these are disadvantages thereof.
Details of the above-mentioned contact mode atomic force microscope may refer to the application for U.S. Pat. No. 5,567,872 with a title of xe2x80x9cScanning Atomic Force Microscopexe2x80x9d submitted by Kyogaku et al., filed on Mar. 7, 1995.
The structure of a known tapping mode atomic force microscope is shown in FIG. 3. The probe tip 1a of the scanning probe 1 scans the surface of the test sample (not shown in the figure). A sinusoidal wave signal generator 10 produces a sinusoidal wave signal that will be sent to a piezoelectricity oscillator 12 to make it vibrate in a manner similar to the sinusoidal wave. This further causes the probe 1 also to move in manner similar to the sinusoidal wave through the transmission of the mechanism. The laser diode 4 outputs a laser beam. The laser beam reflects from the back side of the probe to a photo detector 5 and it will then output a current signal corresponding to the intensity of the laser beam by photoelectric conversion effect. The output current signal will be then converted into a voltage signal by a current/voltage converter 6 and calculated by a differential amplifier 7 to obtain a voltage value corresponding to the amplitude of the probe 1 after the calculation of a digitized signal processor or an analog mean square root signal processing circuit 11. After entering the voltage value to the Z-axis servo controller on the piezoelectricity platform (only the seat is shown), a corresponding control command will be obtained on the basis of the control calculation principle (for example, proportional integral differential, PID) to make the piezoelectricity platform seat 2 move up and down and keep the amplitude of the probe 1 be constant. If the position of Z-axis is recorded at a specific time point and all the data are collected and stored in a scanning image data storing device 9, the surface profile of the test sample can be obtained and displayed on a displayer 19 after being processed by an image data processing device 18. Because the probe tip 1a of the scanning probe contacts only for a considerable short time with the surface of the test sample (not shown in the figure), there is no friction during the scan, which further reduces the surface tension caused by the water molecule on the test sample and the influence of static electricity. Besides, the interference resulting from the vibration of the mechanism itself can be effectively isolated. However, the above-mentioned tapping mode atomic force microscope must have a digitized signal processor or analog mean square root signal processing circuit 11 to obtain the amplitude of AC signals, and the heat deformation caused by continuous laser beams will twist the scanned image. All these are disadvantageous.
Details of the above-mentioned tapping mode atomic force microscope may refer to U.S. Pat. No. 5,412,980 with a title of xe2x80x9cTapping Atomic Force Microscopexe2x80x9d submitted by Elings et al., filed on Aug. 7, 1992.
In the present invention, under the consideration of the fact that the above-mentioned contact mode atomic force microscope must use an interpolation calculation and, therefore, cannot filter the interference caused, for example, by the coaxial light radiated from the current coupling device (CCD) at the scanning probe or the miscellaneous light source, and another, tapping mode atomic force microscope must use a digital signal processor or analog mean square root signal processing circuit to obtain the amplitude of AC signals, and the scanned image will be twisted due to the heat deformation caused by continuous laser beams. It can improve the disadvantages of the above-mentioned contact and tapping modes.
Accordingly, the objective of the present invention is to provide a contact mode atomic force microscope which can reduce the degree of heat deformation caused by laser beams, without the need of interpolation calculation, and can filter the interference caused by the coaxial light radiated from the current coupling device (CCD) or the miscellaneous light source.
Another objective of the present invention is to provide a tapping mode atomic force microscope which can acquire an amplitude of an AC signal by the characteristics of the tapping mode and combining with a low pass filter circuit and can reduce the degree of heat deformation caused by laser beams.
The contact mode atomic force microscope of the present invention uses the pulse signal produced by a pulse signal generator to control, for example, the laser mechanism of modulating laser diodes and output a laser beam in pulse form. The laser beam will reflect from the back side of the probe tip to, for example, a pickup of a laser photo detector in four quadrants and it will then output a current signal corresponding to the intensity of the laser beam. After a conversion from the current signal to the voltage signal is performed, the signal will be calculated by, for example, a magnifying circuit of the differential amplifying circuit. The DC part caused by the coaxial light radiated from the current coupling device (CCD) or the miscellaneous light source and the high frequency harmonic signals caused by the pulse modulation laser beam will be filtered out by a band pass filter. After the modulation/demodulation process made with a mixer/multiplier and the filtration with a low pass filter, a distribution signal having the same frequency as that of the laser before modulating is obtained and, as a consequence, a voltage value corresponding the deformation of the probe can be obtained and uses the value to measure the surface profile of the whole test sample. The present invention uses pulsed laser beams to reduce the heat deformation caused by the laser. The present invention also uses modulation/demodulation process to filter the interference caused by the coaxial light of CCD and other miscellaneous light source. Since the output signal of the present invention is continual, the interpolation operation in unnecessary.
The tapping mode atomic force microscope of the present invention uses the sinusoidal wave signal produced by a sinusoidal wave signal generator and sends it to a piezoelectricity oscillator to bring the probe to move in manner similar to a sinusoidal wave. A phase locked loop produces a pulse modulation signal synchronous with the vibration of the probe to make the laser mechanism (such as laser diode) output a pulse laser beam. The laser beam will reflect from the back side of the probe tip to, for example, a pickup of a photo detector in four quadrants. The current signal corresponding to the intensity of the laser beam will be converted into a voltage signal that will be calculated by, for example, a differential amplifying circuit. The DC part and the high frequency harmonic signal will be filtered out by a band pass filter. After the demodulation process and the filtration of a low pass filter, a voltage value corresponding to the amplitude of the probe can be obtained to keep the amplitude of the probe in a certain value and further measure the surface profile of the whole test sample. Instead of the digital signal processor or analog mean square root signal processor, the present invention uses low pass filter to obtain the amplitude of AC signals. The present invention uses a phase locked loop to produce pulse modulation signals synchronous with the vibration of the probe to make a laser diode output a pulsed laser beam to reduce the heat deformation caused by the laser.
If there is no interference caused by the coaxial light of CCD or miscellaneous light source, this inverted AFM will not use any bandpass filter and demodulation circuit. The voltage value corresponding to the amplitude of the probe can be obtained merely through the filtration of the calculation output of the differential amplifying circuit with a low pass filter.